Method of inhibiting corrosion of iron base metals

ABSTRACT

A process for inhibiting the corrosion of iron based metals in contact with aqueous systems by incorporating in the aqueous system an effective amount of a composition comprising an inorganic phosphate and a water soluble organic copolymer of acrylamido-sulfonic acid monomers with vinyl carboxylic acid monomers.

BACKGROUND OF THE INVENTION

The present invention relates to the inhibiting and preventing corrosionof iron based metals which are in contact with aqueous systems, such ascooling water systems.

Iron and iron metal containing alloys such as mild steel are well knownmaterials used in the construction of apparatus in which aqueous systemscirculate, contact the iron based metal surface and may be concentrated,such as by evaporation of a portion of the water from the system. Eventhough such metals are readily subject to corrosion in suchenvironments, they are used over other metals due to the strength theyhave.

It is known that various materials which are naturally or syntheticallyoccurring in the aqueous systems, especially such systems formed fromnatural resources such as seawater, rivers, lakes and the like, attackiron based metals (the term "iron based metals" shall mean in thepresent disclosure and the appended claims iron metal and metal alloyscontaining iron therein). This is especially true where the aqueoussystem is a hard water i.e. contains about 300 ppm or greater of calciumand magnesium therein.

Typical apparatus in which the iron metal parts are subject to corrosioninclude evaporators, single and multi-pass cooling towers and associatedequipment and the like. As the aqueous system passes through or over theapparatus a portion of the aqueous system evaporates causing aconcentration of the materials contained in the system. These dissolvedmaterials approach and reach a concentration at which they cause severepitting and corrosion which eventually requires replacement of the metalparts.

Various corrosion inhibitors have been previously used. These includeinorganic nitrite or phosphate salts and the like. These materials haveonly minor inhibiting effects and tend to decompose when used inenvirons such as encountered in cooling towers, evaporators and similarapparatus wherein the aqueous system is subjected to elevatedtemperatures.

Copolymers such as described with respect to the present invention havebeen found, when used alone, to have substantially no corrosioninhibiting effect.

It is desired to have a composition and a method capable of being easilyworked which substantially inhibits the corrosion of iron based metals.It is desired to have a composition capable of substantially inhibitingcorrosion of iron base metals of apparatus in contact with aqueoussystems which tend to concentrate. It is further desired to have acomposition which is capable of inhibiting corrosion when used at verylower dosages.

SUMMARY OF THE INVENTION

The present invention is directed to a method of inhibiting corrosion ofiron based metals which are in contact with aqueous systems by mixingwith the aqueous system a threshold quantity of an inorganic phosphateand a water soluble organic copolymer formed from acrylamido-sulfonicacid monomers and vinyl carboxylic acid monomers.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention it has been surprisingly found thatthe desired corrosion inhibition can be achieved by the use of aspecific composition. This composition is the combination of aninorganic phosphate and certain organic copolymers as described indetail hereinbelow. It has been found that the subject combination ofcomponents results in a synergistic desired effect.

Accordingly, the present invention provides a method of inhibitingcorrosion of iron base metals in contact with an aqueous system byincorporating into the aqueous system a water soluble inorganicphosphate compound e.g. orthophosphoric acid, alkali metal phosphates,such as sodium or potassium orthophosphates, sodium or potassiumpyrophosphates, sodium or potassium metaphosphates sodium or potassiumtripolyphosphate, sodium or potassium hexametaphosphate and the like.The phosphate compound should be water soluble. The preferred salts arethe sodium salts.

The copolymeric material required to be used in combination with theinorganic phosphate described above can be represented by the generalformula ##STR1## wherein R¹ and R² each independently represent hydrogenor methyl; R³ represents hydrogen or C₁ -C₁₂ straight or branch chainalkyl group, preferably a C₁ to C₃ alkyl group, or a cycloalkyl grouphaving up to six carbon atoms or a phenyl group; M represents hydrogenor an alkali metal cation or alkaline earth metal cation or an ammoniumcation or mixtures thereof selected from metal or ammonium cations whichpresent no adverse effect to the polymer solubility in water, thepreferred cations are selected from alkali metals, and ammonium cationswith sodium, potassium and ammonium being most preferred; Z representshydrogen or an alkali metal or ammonium cation or mixtures thereof; xand y are integers such that the ratio of x to y is from about 5:1 to1:5 and the sum of x+y is such that the copolymer has a weight averagemolecular weight of between 1,000 and 100,000 and more preferablybetween 1,000 and 10,000 and most preferably between 4,000 and 6,000.

The preferred copolymers are formed from acrylic acid or methacrylicacid or their alkali metal salts in combination with2-acrylamido-2acrylamido-2methylpropane sulfonic acid or its alkalimetal or ammonium salts. The copolymers can be partially or completelyneutralized as the salt. The molar ratio of the monomeric material isfrom 5:1 to 1:5 and preferably from 2:1 to 1:2.

The copolymer required for use in the composition of the subjectinvention may contain minor amounts of up to about 5 mole percent ofother monomeric units which are inert with respect to the subjectprocess such as lower (C₁ -C₅) esters of acrylic or methacrylic acid,acrylonitrile and the like.

The copolymer required for forming the composition found useful inperforming the subject process can be formed by conventional vinylpolymerization techniques. The monomers of 2-acrylamido-2-methylpropanesulfonic acid, methacrylic acid and acrylic acid (as appropriate) areeach commerically available. The monomers are mixed in appropriate molarratios to form the desired product and are polymerized usingconventional redox or free radical initiators. Formation of lowmolecular weight copolymers may require the presence of chainterminators such as alcohols and the like in manners known in the art.

In general the phosphate and copolymer are used in weight ratios of from100:1 to 1:100 and more preferably from 4:1 to 1:4 and most preferablyabout 1:1.

The dosage of the composition of the present invention depends, to someextent, on the nature of the aqueous system in which it is to beincorporated. In general however, it can be said that the concentrationin the aqueous system can be from 1 to 200 ppm although much lowerdosages of from 1 to 100 ppm is normally sufficient and even lowerdosages of from 1 to 25 ppm substantially inhibits corrosion. The exactamount required with respect to a particular aqueous system can bereadily determined in conventional manners.

The composition may be added to the aqueous system coming in contactwith the metal surfaces of an apparatus by any convenient mode, such asby first forming a concentrated solution of the composition with waterand then feeding the concentrated solution to the aqueous system at someconvenient point in the operation. Alternately, the above-describedphosphate and copolymer can be each separately added directly to theaqueous system to allow the formation of the subject composition to formin situ in the aqueous system. It is believed, although not made alimitation of the instant invention, that the copolymer and inorganicphosphate interact to attain the achieved corrosion inhibition whichresults are not attainable by use of each of the individual components.It is known that the phosphates disclosed herein have only a fair degreeof corrosion inhibiting effect and that the copolymers described hereinhave substantially no corrosion inhibiting effect. The two components,when used in concert, causes and provides a substantial corrosioninhibiting effect.

It will be further appreciated that other ingredients customarilyemployed in aqueous systems of the type treated herein can be used inaddition to the subject composition. Such water treatment additives are,for example, biocides, lignin derivatives and the like.

The following examples are given for illustrative purposes only and arenot meant to be a limitation on the subject invention except as made inthe claims appended hereto. All parts and percentages are by weightunless otherwise indicated.

EXAMPLE I

A series of tests were performed using a dynamic test apparatus whichsimulates conditions encountered in a recirculating cooling tower.

The apparatus comprises a vertical glass cylinder having an eightkilowatt cylindrical stainless steel cartridge heater inside. Thecylinder was closed with a ballcock at the top and was equipped withtemperature measurement probes at the inlet and outlet ports. Thecylinder further contained a chamber in which metal (mild steel) samplecoupons could be placed. An open plastic vessel having a five litercapacity was used as the open reservoir. This reservoir had an outlettube connected to a centrifical pump which fed (via a rotometer) intothe bottom inlet port of the cylinder. The outlet port is connected tothe reservoir to return the aqueous fluid thereto. The reservoir alsocontains a cooling coil to maintain the reservoir water at 130° F. and awater make-up actuated by a diaphragm pump. The apparatus was maintainedat a circulation rate of 1.5 gallon per minute with an inlet temperatureof 130° F., and a pH of 7.7±0.2.

The aqueous systems which circulated through the dynamic test apparatuswere synthetic hard water solutions containing 635 ppm calcium sulfatehemihydrate, 518 ppm magnesium sulfate heptahydrate, 136 ppm calciumchloride, 632 ppm sodium chloride, 816 ppm sodium sulfate, 64 ppm sodiummetasilicate and 185 ppm sodium bicarbonate. In addition the aqueoussystems were dosed with a copper corrosion inhibitor (benzotriazole) anda phosphonate sequestering agent (hydroxyethylidene-1, 1-diphosphonicacid).

To each of the test solutions was added a composition comprising aninorganic water soluble phosphate component in the form of 7.5 ppmactive H₃ PO₄ and 8 ppm sodium tripolyphosphate (6.3 ppm as H₃ PO₄) andvarying amounts (see Table I below) of a copolymer (MW_(w) =5000) of2-acrylamido-2-methylpropane sulfonic acid and methacrylic acid (1:2) asthe sodium salt. Duplicate clean mild steel coupon specimens wereweighed and placed in the coupon specimen chamber to be subjected to aflow of the aqueous system at a rate of 1.5 gallon per minute for aperiod of 10 days. At the end of the 10 day test period, the steelspecimens were removed, cleaned free of deposits, washed and dried. Thespecimens were then weighed to determine corrosion loss.

                  TABLE I                                                         ______________________________________                                        Phosphate                                                                     (as H.sub.3 PO.sub.4)                                                                  Copolymer Corrosion Rate                                             ppm      ppm       milli-inch per year                                                                         Improvement                                  ______________________________________                                        13.8     --        7.4           --                                           13.8     4.5       5.8           22%                                          13.8     7.0       3.6           51%                                          13.8     9.0       4.0           46%                                          13.8     10.0      2.2           70%                                          ______________________________________                                    

A control was conducted as described above in which the phosphatecomponents were added to the aqueous system without the addition ofcopolymer. The corrosion rate was calculated to be 7.4 milli inch peryear. It is known that the subject copolymer does not exhibit corrosioninhibition yet Table I shows that the subject combination unexpectedlygives increased corrosion inhibition in comparison to the use of justphosphates.

EXAMPLE II

Repeat series of the above tests are conducted in the same manner asdescribed in Example I above except that the (a) copolymer is in theform of the free acid, (b) the copolymer is formed from sulfonicacid/carboxylic acid monomer in 1:1 mole ratio and (c) an equivalent ofsodium hexametaphosphate is used in lieu of the phosphates used above.

While the invention has been described in connection with certainpreferred embodiments, it is not intended to limit the invention to theparticular forms set forth, but, on the contrary, it is intended tocover such alternatives, modification and equivalents as defined by theappended claims.

What is claimed is:
 1. A process of inhibiting corrosion of iron basedmetals in contact with a cooling water system comprising incorporatinginto the cooling water system from 1 to 200 ppm of a compositioncomprising (A) a water soluble inorganic phosphate and (B) a copolymerof 2-acrylamido-2-methylpropane sulfonic acid and methacrylic acid in amolar ratio of from about 5:1 to 1:5, said copolymer having a weightaverage molecular weight of between 1,000 and 100,000; an alkali metal,alkaline earth metal, or ammonium salt of said copolymer; or a mixtureof said salts; said composition having a weight ratio of component (A)to component (B) of from 100:1 to 1:100.
 2. The process of claim 1wherein the copolymer has a weight average molecular weight of between1,000 and 10,000.
 3. The process of claim 1 wherein the water solubleinorganic phosphate is orthophosphoric acid or an alkali metalphosphate.
 4. The process of claim 1 whrein the copolymer has a weightaverage molecular weight of from 4,000 to 6,000; the molar ratio of2-acrylamido-2 methylpropane sulfonic acid to methacrylic acid is from2:1 to 1:2; and the weight ratio of component (A) to component (B) isfrom 4:1 to 1:4.
 5. The process of claim 1 wherein the copolymer has amolar ratio of 2-acrylamido-2-methylpropane sulfonic acid to methacrylicacid of from 1:1 to 1:2.